Printed circuit with thermal drain

ABSTRACT

The invention discloses a printed circuit card on which is welded at least an electronic component box as well as a thermal dissipator for dissipating the heat emitted by the component box. The component box and the thermal dissipator are respectively provided with clips and lugs which are engaged and welded in respective holes drilled in the card. A thermal drainage metallic area provided on a zone of the card located under the box forms a thermal adduction path between the box and the lugs of the thermal dissipator. This thermal drainage metallic area is further connected to a clip ensuring a privileged thermal drainage of the electronic component box.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a printed circuit card carrying in particularcomponents boxes positioned and fixed by welding, as well as dissipationmeans of the heat emitted by the boxes.

Such a card comprises:

a mounting face for the boxes,

a welding face of the boxes,

conductive tracks provided on the mounting face and on the welding face,

holes connecting tracks of the one and the other face, the boxes beingprovided with connecting clips inserted in the holes, and

a thermal dissipator which is mounted on the card for dissipating theheat emitted by the components boxes.

2. Description of the Prior Art

In order to avoid the problems related to the compensation of backlashesencountered with the dissipators usually mounted directly on the boxes athermal linking path is provided for connecting the boxes to thedissipator. It is observed in practice that the electronic componentspresently available on the market, which are positioned on a printedcard withstand temperatures higher than the card. It is thus essentialto improve the calorific exhaust efficiency permitted by the dissipatorand by the thermal path connecting the boxes to this dissipator, if thetemperature of the card must be kept lower than a given value, e.g.typically around 105° C.

It is known, according to the French patent No. 2560731 to providebetween a component box and the card which carries it, a copper plateconstituting a thermal shunt which can extend up to the dissipator. Aconduction thermal link is set up between the plate and the box by meansof a plastic ceramic strip. The construction of such a cooling system isobviously intricate.

SUMMARY OF THE INVENTION

It is, therefore, an object of this invention to provide an efficientcooling device on a card of the above mentioned type.

It is another object of this invention to simplify the constructionprocess of the cooling device specially without complicating the generalmanufacture process of the card.

According to the invention, a thermal drainage metallic area is providedon at least one of the card facesin order to collect a part of the heatemitted by each box, this area covering at least a part of the zone ofthe said face located under the box; the thermal dissipator isconstituted by a metallic piece with lugs inserted in the card in closevicinity to the clips of at least one box, the drainage area forming athermal adduction path between the respective box and the dissipator.

The dissipator which is constituted by a metallic piece with lugs canthus be inserted on the card in the same way as the electroniccomponents and can be welded to the card by the wave passage operationprovided for the components welding. It is observed that theconstruction of the cooling system is thus very simple. In order toreduce the stresses exerted on the card after the wave passage,provision can be made advantageously on a card carrying a multiplicityof boxes of rather than one dissipator, several dissipators each onebeing associated to one or serveral boxes.

The drainage area may be provided on the mounting face in order tocollect the heat radiated by the boxes, the drainage area is thenpreferably isolated from the box by an air space, thus enabling to avoidthe difficulties due to the positioning of the box on this area. Thedrainage area may also be provided on the welding face. The drainagearea extends preferably, on the one hand up to a hole provided for a boxclip, assuring preferably a privileged thermal drainage, and on theother hand up to at least a hole provided for one lug of the dissipator.

The drainage area is constituted preferably by a metallic layer obtainedone coated with a passivation layer, for instance an insulating varnish,through the same masking, etching and insulation coating operationswhich enable the achievement of the interconnection tracks. In whatconcerns the drainage area located on the welding face side, this areahas also a sufficiently large surface in order to assure a thermaldistribution or homogeneisation function with the aim of avoiding theoccurrence of hot spots on the card and it is preferably sufficientlythickened by a tinning layer subsequent to the wave passage of thewelding face of the card.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the following descriptionof a preferred embodiment with reference to the to the drawings inwhich:

FIG. 1 is a diagrammatic cross-section of a part of a printed circuitedcard according to the invention,

FIG. 2 is a side view, partially in cross-section, along line II--II ofFIG. 1,

FIG. 3 represents on a larger scale the detail A of FIG. 2,

FIG. 4 is an exploded perspective view of the card according to theinvention,

FIGS. 5 and 6 are respectively an upper and lower view of a part of thecard in the vicinity of a box.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The printed circuit card B shown on the figures is actually aninsulating material plate 10 coated with copper on its two faces, vizusa face 11 for mounting the electronic components C, specially of thedual-in-line package DIL type and a welding face 12 for thesecomponents.

The faces 11 and 12 carry interconnection tracks 13, obtained in aconventional way by selective etching of the copper coating. Holes 14drilled in the card and coated with a metallization 15 are provided forlocating connecting clips 16 of the components C boxes 17. These latterare manually or automatically positioned on the card B and are fixedthereto by welding wave V passage under the face 12 (see FIG. 2).

According to the invention, a thermal dissipator S is located on thecard and thermally coupled to one or several boxes through at least onethermal drainage metallic area.

The dissipator S is constituted by a metallic piece, for instance intinned copper, it is an L-shaped piece with openings 20 enabling thepassage of the cooling air and with cut-out lugs 21 inserted in holes 22of the card. The holes 22 are metallization coated 23; they arecylindrical and of a diameter larger than the holes 14 assigned to theclips 16 of the boxes, the lugs 21 having a cross-section larger thanthat of the clips 16. On the cross-section of FIG. 1, the central lug 21of the dissipator has been shown to represent the device, although it isshifted forward with respect to the clips 16 as shown on FIG. 2. On theother hand, only one of the clips 16 of the box has been shown on FIG.2.

After being positioned on the card, the dissipator S is fixed thereto bythe wave passage V operation, which enables to fix the components C(FIG. 2). Instead of being made of a cut-out piece, the dissipator canbe made of a moulded piece.

Two metallic areas 24 and 25 for thermal drainage are respectivelylocated on the mounting face 11 and on the welding face 12 of the card.The area 24 covers a part of the zone of the face 11 underlying the box17 (see FIGS. 2, 4 and 5) being isolated from the box by an air space 26(see FIG. 1), in order to collect the heat emitted by the bulge of thebox.

The area 25 covers the zone of the face 12 underlying the box andextends beyond this zone (in 25a, FIGS. 2, 4 and 6) to assure anefficient distribution of the thermal flux on the card surface, in orderto avoid the occurrence of hot spots on this latter.

It is noteworthy that at least one clip 16' of the box 17 is linked byconduction, on the one hand to the area 24 and on the other hand, to thearea 25. This clip 16' is preferably a privileged thermal dissipationclip of the component, for instance solidly linked to the frame (leadframe) of the chip contained in the box.

The connection of the clip 16' with the area 24 is carried out thanks tothe welding upward extension 27 which fills the hole 14 and to themetallization 15 thereof being itself in contact with the area 24. Theconnection of the clip 16'-with the area 25 is carried out by means ofthe welding 27 and of the metallization 15 which is in contact with thearea 25 (see FIG. 3).

The areas 24 and 25 extend up to the metallizations 23 of the holes 22associated to the lugs 21 of the dissipator S. As shown on FIG. 3, thearea 25 is thickened by a metallic layer 25b, specially tin, by wavepassage, thus enabling to ascribe thereto a cross-section of sufficientsize and to emphasize its function of thermal distribution.

In the present embodiment, the card includes several dissipators S eachone being associated to several components C aligned on the card. Toeach component C correspond three drainage lugs 21 of the dissipator(FIGS. 1 and 4-6). The dissipator lugs 21 are positioned in closevicinity to the clips 16 of the components, being aligned at right anglewith the alignment of clips 16.

It must be observed that metallic areas of drain 24, 25 are obtained bymasking steps and selective etching steps, then coated with apassivating layer in the course of the process and of passivatingcoating of the interconnection tracks.

What is claimed is:
 1. A printed circuit card carrying at least anelectronic component box having connecting clips which are welded on thecard, said card presenting a mounting face for the box, a welding faceof the box, conducting tracks provided on the mounting face and on thewelding face, a first plurality of metallized holes wherein theconnecting clips are inserted and welded and a thermal dissipatormounted on the card for dissipating the heat emitted by the box,wherein:i- a thermal drainage metallic area collecting part of theheating emitted by the box is constituted by a zone of the welding facelocated under the box which is covered by a thick layer of solderingmetal; ii- the thermal dissipator is a metallic piece having lugsrespectively positioned and welded in a second plurality of metallizedholes provided on the card in close vicinity to at least one determinedconnecting clip of said box, said lugs being also welded on said facesof the card; iii- the thermal drainage metallic area forms a privilegedthermal adduction path between said determined connecting clip and thelugs of the thermal dissipator.
 2. A printed circuit card as claimed inclaim 1, wherein the card carries several boxes and several dissipatorsare positioned on the card each dissipator being thermally associatedwith at least one box.
 3. A printed circuit as claimed in claim 1,wherein the thermal dissipator presents several lugs thermallyassociated with the said box.
 4. A printed circuit card as claimed inclaim 1, wherein the holes of the second plurality have a diameterlarger than the holes of the first plurality and the lugs have across-section large than that of the connecting clips.
 5. A printedcircuit card as claimed in claim 1, wherein the lugs are aligned in afirst direction and the clips are aligned in a second direction forminga right angle with the first direction.
 6. A printed circuit card asclaimed in claim 1, wherein the thermal drainage metallic area islocated under a zone of the mounting face collecting the heat radiatedby the box and being isolated form the box by an air space.